The present invention relates to improvements in load pushing and push-pull assemblies for lift trucks, and in particular to improvements in mechanisms for controlling the angular attitude and vertical position of pushing or push-pull frames.
In the lift truck handling of certain types of loads, such as unpalletized boxes or stacks of cartons, it is common procedure to utilize a selectively extensible and retractable push frame atop a tiltable load-supporting frame (such as load forks or a platen) so that the load may be deposited by tilting the load-supporting frame downward and pushing the load forwardly off of the tilted frame onto the floor through extension of the push frame. During the pushing motion, it is desirable to apply the major thrust near the bottom of the load in order to avoid pushing deformable stacks of cartons out of vertical alignment.
Push frames are commonly used in conjunction with a selectively openable and closeable clamp adjacent the bottom of the push frame for gripping the edge of a slip sheet positioned beneath the load and, by retraction of the push frame and clamp, pulling the load onto the downwardly tilted load-supporting frame. When the push frame is equipped with such a clamp, the assembly is normally referred to as a push-pull assembly since it is capable of accomplishing two functions rather than merely the pushing function. The pulling function, however, is rendered difficult in most push-pull assemblies because the downward tilt of the load-supporting frame, necessary to pick up the load, also dictates that the push frame assume an angular attitude other than perpendicular to the warehouse floor or other surface supporting both the load and the lift truck. This is due to the fact that push frames have, in the past, always been supportably mounted with respect to the load-supporting frame such that, when the angular attitude of the load-supporting frame is changed by tilting it, the angular attitude of the push frame is likewise changed because the load-supporting frame and push frame tilt in unison. This tilting in unison occurs regardless of whether the truck is of the high-lift type, as shown for example in Anderson, Jr. U.S. Pat. No. 3,885,692 and Brudi U.S. Pat. No. Re. 28,623, or is a low-lift truck of the type shown in Rigsby U.S. Pat. No. 3,495,730 and Rocco U.S. Pat. No. 4,065,012.
With structures such as those shown in the Anderson, Jr. and Rocco patents, the push frame is tilted forwardly and downwardly when the load-supporting frame is tilted downwardly to pick up or deposit a load. This tilting of the push frame positions the bottom of the push frame, when extended, in close proximity with the forward end of the downwardly tilted load-supporting frame, which is particularly necessary in order to depress the fixed bottom jaw of a slip sheet clamp sufficiently to engage the edge of a slip sheet. However, when the push frame is extended for either the pushing or pulling function, the push frame is askew with respect to the vertical side of the load causing deformation of deformable vertical stacks and, because the upper end of the push frame encounters the upper end of the vertical side of the load before the clamp is close enough to the bottom of the load to grasp the slip sheet, severely hampering the pulling function.
This problem is partially solved by a structure such as that shown in the above-identified Brudi patent wherein the geometry of the extension and retraction linkage is such as to forcibly tilt the push frame rearwardly upon extension so as to compensate for the downward tilt of the load-supporting frame and push frame assembly. However verticality with respect to the truck-supporting surface can be achieved by this means in only one particular tilting angle of the load-supporting frame, and not at all tilting angles, because the push frame still tilts in unison with the load-supporting frame. This same disadvantage is true of the device shown in the Rigsby patent.
In handling slip sheet-supported loads, it is often necessary to pick up or deposit the load while the main frame of the truck is supported on a surface which is elevated or depressed with respect to the surface upon which the load is supported or is to be placed. This can occur for example at the interface between a loading dock and a truck bed, or when a slip sheet-supported load is placed upon a pallet. Under these circumstances the tilting attitude of the load-supporting frame with respect to the main frame of the lift truck will vary greatly when picking up or depositing a load. In fact in those cases where the load-supporting surface is higher than the truck-supporting surface, the load-supporting frame may not be tilted downwardly at all during load deposit or pick up. Accordingly those assemblies which achieve verticality of the push frame in the extended position, but only at a particular tilting attitude of the load-supporting frame, provide inadequate flexibility of the truck to satisfy the varied load-handling situations likely to be encountered.
What is needed therefore are load-pushing and push-pull assemblies which maintain the push frame in a substantially constant angular attitude with respect to the truck-supporting surface, especially in the extended position of the push frame, in all tiltable positions of the load-supporting frame.